Is this school ___ you visited last year?A.the one B.that C.where D.which 为什么选A而不选其他选项？

变成陈述句就简单了：
This school is ____ you/I visited last year.

如果直接选关系词引导定语从句的话，则主句为This school is.
主句都不完整，这是不符合英语语法的。

只有选A，先给主句加上表语及给定语从句加上先行词才可以。
This school is the one (which/that) you/I visited last year.
因为关系代词which/that代替the one即the school，在定语从句中做visited的宾语，所以which/that可以省略不写。

所以答案A为正解。

转变成陈述句就是this is the school that ....所以题目中如果有冠词的话就可选that/which了,现在没有就用the one来代替了

把这句话变为陈述语序

this school is ____ you visited last year.主语为this school 系动词是is 。the one作表语。那么后面的you visited last year 就是定语，修饰前面的the one